The parent applications show a method and apparatus for minimizing adverse environmental impact of the production of kraft pulp, or the like, by bleaching the pulp without chlorine or chlorine-containing liquids, and removing metals (such as Mn, Fe and Cu) to prevent buildup of those metals to a level adversely affecting the bleaching reactions. Basically, according to the parent applications an acidic liquid stream, such as washer filtrate following a bleach plant acidic stage, is treated with alkaline material containing carbonate in a reactor so that the pH is raised to a point where much of the dissolved metal in the filtrate precipitates out, and the precipitated metals are removed by optional clarification and/or filtration. The carbonate primarily controls the calcium level of the filtrate, but also provides pH control and aids in the precipitation of Ca, Mn, etc. The parent applications also show a method and apparatus for oxidizing the filtrate to increase the valence of the metals (like Mn and Fe) so that they become easier to precipitate or flocculate.
In the parent applications, preferred alkali liquids for increasing the pH of acidic bleach plant effluents so that the metals dissolved in the effluents readily precipitate out of solution are green liquor and white liquor. While both of those liquors are excellent mediums for facilitating precipitation of the dissolved metals, and are readily available in the mill, if the metal-depleted filtrate produced thereby is to be used in the bleach plant of the pulp mill, there can be disadvantages associated with the use of green liquor or white liquor. Both green liquor and white liquor contain undesirable amounts of bleaching chemical-consuming sulfides, such as dissolved hydrosulfide ions (HS.sup.-) and thiosulfate (S.sub.2 O.sub.3.sup.2-). In order to avoid the consumption of bleaching chemical by sulfides, yet retain all of the advantages (such as carbonate ion addition) associated with the use of green liquor or white liquor as the alkaline liquids for effecting precipitation, according to the present invention, a simple, advantageous, yet effective process and apparatus are employed for removing the chemical-consuming sulfides and increasing the carbonates so that the filtrate can be used anywhere in the pulp mill, including in the bleach plant (e.g. as a wash liquid, to manufacture bleaching chemical, etc.).
According to the present invention, the green liquor or white liquor is treated with carbon dioxide gas in conventionally available equipment. Carbon dioxide gas is readily available as a by-product of combustion processes already existing in the mill, such as in the recovery boiler, hog-fuel boiler, or lime kiln. Also, carbon dioxide is readily available commercially. By treating the green liquor or white liquor with carbon dioxide gas according to the invention not only are the sulfides removed therefrom, the carbonate content of the liquid increases, which is very desirable, since it reduces the solubility of manganese and calcium so that those metals will more readily precipitate when the pH of the effluent is increased. Also, the additional carbonate extends, or lowers, the pH range in which the metals will precipitate.
According to one aspect of the present invention, a method of removing bleaching chemical-consuming metals from treatment fluids in a cellulose pulp mill having a bleach plant is provided comprising the steps of substantially continuously: (a) Treating digested pulp in an acidic or neutral stage to dissolve metals. (b) Washing or thickening the pulp after step (a) to produce a filtrate containing dissolved metals. (c) Treating an alkali liquid containing bleaching-chemical-consuming sulfides with carbon dioxide to reduce the sulfide content and to increase the carbonate content of the alkali liquid. (d) Using the alkali liquid produced in step (c) to adjust the pH of the filtrate from step (b) to cause dissolved metals to precipitate as solids in the pH adjusted filtrate. (e) Removing the precipitated solids from the filtrate. And, (f) using the reduced metal content filtrate from step (e) in the bleach plant or wherever it is needed in the pulp mill.
Step (c) is desirably practiced by treating green liquor or white liquor so as to remove dissolved hydrosulfide ions therefrom, by absorbing CO.sub.2 gas into the liquid such as by scrubbing the liquor with carbon dioxide gas (typically using a plate column scrubber, a stationary packing scrubber, or like commercially available gas-liquid contacting equipment).
Step (e) is typically practiced by filtration, flotation, or sedimentation. Also there may be the further step of oxidizing the filtrate from step (b) prior to step (d) in order to increase the valence of the dissolved metals so that they become easier to precipitate or flocculate. The carbon dioxide gas used in step (c) is typically produced during combustion in a recovery boiler, hog-fuel boiler, or lime kiln of the pulp mill, although it can be purchased from an outside source. If the filtrate is not oxidized, step (d) is practiced to adjust the pH so that it is in the range of about 8-11, while if the filtrate is oxidized, the pH in step (d) is adjusted so that it is in the range of about 6-11. Steps (a) through (f) may be practiced to produce a filtrate having a carbonate level of at least about 0.05 moles per liter.
According to another aspect of the present invention a method of treating digested kraft pulp is provided comprising the following steps: (a) Oxygen delignifying the kraft pulp. (b) Treating the oxygen delignified pulp in an acidic stage. (c) Washing or thickening the pulp after step (b) to produce a filtrate. (d) Treating green liquor or white liquor containing bleaching chemical-consuming sulfides with carbon dioxide to reduce the sulfide content and to increase the carbonate content of the liquor. (e) Using the liquor from step (d) to adjust the pH of the filtrate from step (c) to cause dissolved metals to precipitate as solids in the pH adjusted filtrate. And, (f) removing the precipitated solids from the filtrate.
The invention also relates to an apparatus for removing bleaching chemical-consuming metals from treatment fluids in a cellulose pulp mill having a bleach plant. The apparatus comprises the following elements: Means for treating digested pulp in an acidic or neutral stage to dissolve metals. Means for washing or thickening the pulp from the treating means to produce a filtrate containing dissolved metals. Means for treating an alkali liquid containing bleaching chemical consuming sulfides with carbon dioxide to reduce the sulfide content and to increase the carbonate content of the alkali liquid, the means comprising means for absorbing the carbon dioxide gas into the alkali liquid (e.g. by scrubbing). A reactor for combining the alkali liquid, produced by the means for treating an alkali liquid, with the filtrate to adjust the pH of the filtrate to cause dissolved metals to precipitate as solids in the pH adjusted filtrate. And, means for removing the precipitated solids from the filtrate.
It is the primary object of the present invention to facilitate the effective removal of metals from treatment fluids in cellulose pulp mills which adversely affect the bleaching reactions, while at the same time producing a treated filtrate that is optimal for use anywhere in the pulp mill including the bleach plant. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.